On the subject of Marvel Mystery Oil…I’ve been in the engine business for 30 years now and during this time have been privileged to a great deal of test data on various lubricants, oil additives, and other associated products. During my tenure with Mercury Marine, we tested oils and additives for both two-stroke and four-stroke engines. Claims made for power and economy improvements by reducing friction were all proven bogus by careful dynamometer testing under controlled conditions. Those expensive products containing Teflon were particularly interesting in that they did absolutely nothing for performance. Synthetic lubricants, such as Mobil 1, would show slight gains in performance mainly due to the ability to run a lighter viscosity oil safely without oxidation occurring. The synthetic oils were legitimate, the other products were not.
There was one product, however, that was unique and did what its makers claimed…that was Marvel Mystery Oil, commonly referred to on this forum as MMO. Intended as an oil or gasoline additive, it has the ability to soften hard carbon deposits that tend to form in air-cooled engines. Particularly vulnerable to carbon induced problems are exhaust valve stem-guide seizures, piston ring sticking, and carbon build-up at the top of the cylinder bore and combustion chamber area. The high temperatures in which an air-cooled engine operates (compared to the liquid-cooled counterpart) seem to be the root cause of this type of deposit fomation. Due to the additives in MMO, these deposits are softened and are washed away by either the crankcase lubricant, or are carried out by the exhaust gasses. There have been instances where an oil-burning engine was “reconditioned” by MMO, simply by freeing up carbon stuck piston rings. Exhaust valve sticking can also be reduced or eliminated by using MMO mixed with the gasoline in the ratio stated on the bottle. On a new engine, or one that’s in good condition, a steady diet of MMO in the fuel is an excellent preventative for any carbon-related problems that might occur during the operational life.
Probably the best recommendation for MMO comes from aircraft A&P’s (airframe and power plant mechanics) who highly recommend it. I talked to one guy who wrenched for a freight operator, flying Douglas DC-3’s. He claimed their engines (Pratt & Whitney R-1830, 14-cylinder, 1200 HP air-cooled radials) would always make their TBO (time between overhauls) without incident, due partly to running MMO in the fuel. This impressed me.
MMO has proven to work well in the Kohler K-series engines that power our Cubs. My two get a steady diet of it in their fuel, mixed at the recommended ratio. As an added benefit, the exhaust gasses smell quite nice.
